Wisconsin homeowners and city foresters have their hands full; trees line the streets of their municipalities and fill vast areas of the interior in state and national forests, parks and wildlife refuges. The Department of Natural Resources and city foresters trim trees on public property, but homeowners are responsible for pruning trees on their own property to keep them healthy and safe.
Winter in Wisconsin
The Wisconsin Department of Natural Resources (DNR) and University of Wisconsin Extension Service (UW Extension), like many authorities, advise pruning and trimming trees during dormancy; in other words, winter.
The problem in Wisconsin is to define winter in terms of what part of the state you want to trim a tree. "Winter" begins much sooner in northern Wisconsin where the first average frost date occurs in the middle of September, than in the south part of the state where the first frost occurs as late as the end of October along Lake Michigan. By Halloween, the northern end of the state may have had its first snow and "trackers" snow flies in the south as early as Thanksgiving. As for late-winter pruning, the challenge is to beat the sugar maples. Horicon Marsh tree-owners had best get started in February; the geese are back and forsythia is blooming by mid-March on the south end. Christmas tree growers in Bayfield County, however, have most of April to trim their crops.
Because there are so many maples in Wisconsin--the sugar maple is the state tree--the DNR and UW Extension both recommend early winter early rather than late winter pruning to guarantee that the trees are dormant. UW Extension also recommends pruning oak trees in late fall to early winter to prevent the spread of oak wilt.
According to the University of Minnesota, oak species most likely to be affected by oak wilt include Northern red oak and pin oak. Burr oaks are less sensitive and white oaks have some resistance. All are species that grow in southern and western Wisconsin. Prune in the summer after leaves have fully formed only when it is necessary for the health of the tree or the safety of the surrounding area. Never prune the river birch, another Wisconsin native, between May 1 and August 1 to minimize exposure to the bronze birch borer.
UW Extension recommends pruning the state's conifers (firs, cedars, hemlock, juniper, pine, spruce, tamaracks and yews) in late winter before new candles appear.
The DNR suggests pruning deciduous trees from November to March. The onset of winter can be sudden and severe and the passage of spring hardly noticeable (but also severe) in many parts of the state. In Wisconsin's apple orchards, summer trimming after blossoming controls tree shape and maximizes the next year's crop. The same is true of the other fruits grown in the state; Asian pears, cherries, nectarines, peaches and plums. As with any fruit tree, Wisconsin orchard growers shape young trees carefully when they are young, completing major trimming during early winter.